import c-shared library that generated by cython to go with cgo

Question

I wanna import a c-shared-library to go that generated by Cython in python 3.7, try do it by cgo.

in this case:

go version go1.12.7 linux/amd64

Python 3.7.3

Cython version 0.29.12

os: Manjaro 18.0.4

Kernel: x86_64 Linux 5.1.19-1

I will continue: make a python file vim pylib.pyx:

#!python
cdef public void hello():
     print("hello world!")

and run python -m cython pylib.pyx for generate the c-shared-library, I have two files, pylib.c and pylib.h. now, try import these to golang, so make a go file vim test.go:

package main

/*
#include </usr/include/python3.7m/Python.h>
#include "pylib.h"
*/
import "C"
import "fmt"

func main() {
   C.hello()
   fmt.Println("done")
}

finaly, I run go run test.go: I have the following output:

# command-line-arguments
/usr/bin/ld: $WORK/b001/_x002.o: in function `_cgo_51159acd5c8e_Cfunc_hello':
/tmp/go-build/cgo-gcc-prolog:48: undefined reference to `hello'
collect2: error: ld returned 1 exit status

I try import it to c too but I encountered a similar output like this:

undefined reference to `hello'
ld returned 1 exit status

I don't know what to do, help me, please. :(

Answer 1

I run go run test.go: I have the following output:

# command-line-arguments
/usr/bin/ld: $WORK/b001/_x002.o: in function `_cgo_51159acd5c8e_Cfunc_hello':
/tmp/go-build/cgo-gcc-prolog:48: undefined reference to `hello'
collect2: error: ld returned 1 exit status

---

We can generate an equivalent error message with the following code.

package main

/*
#include <math.h>
*/
import "C"
import "fmt"

func main() {
    cube2 := C.pow(2.0, 3.0)
    fmt.Println(cube2)
}

Output:

$ go run cube2.go
# command-line-arguments
/usr/bin/ld: $WORK/b001/_x002.o: in function `_cgo_f6c6fa139eda_Cfunc_pow':
/tmp/go-build/cgo-gcc-prolog:53: undefined reference to `pow'
collect2: error: ld returned 1 exit status
$ 

In both cases, ld (the linker) can't find a C function after looking in the usual places: undefined reference to 'pow' or undefined reference to 'hello'.

Let's tell cgo where to find the C pow function in the C math library: m.

For cgo, using ld flags,

#cgo LDFLAGS: -lm

GCC: 3.14 Options for Linking

-llibrary
    Search the library named library when linking.

Updating the previous code,

package main

/*
#cgo LDFLAGS: -lm
#include <math.h>
*/
import "C"
import "fmt"

func main() {
    cube2 := C.pow(2.0, 3.0)
    fmt.Println(cube2)
}

Output:

$ go run cube2.go
8
$

---

This illustrates a basic cgo principle: include a C header file for your C library and point to the location of the C library.

---

References:

Cgo and Python : Embedding CPython: a primer